Core flood testing of phosphonate-based scale inhibitors in reservoir sandstone containing iron minerals revealed potential formation damage caused by unmodified acid phosphonates, which are reactive toward the iron minerals. The mechanism of formation damage was shown to be due to dissolution and re-precipitation of iron phosphonate species, together with fines mobilized (also thought to be due to dissolution effects) which propagated some distance through the core, leading to pore throat blockages. In an example case, complete blockage occurred as the front of the injection chemical reached the outlet of the core due to a combination of silica fines and iron phosphonate chemical. To mitigate the potential issue, different chemical formulations were prepared based on the same phosphonate scale inhibitor, in order to mitigate / minimise the potential for in-situ dissolution of iron bearing minerals within the reservoir formation. This allowed a less aggressive formulation to be prepared by part-neutralisation of the acid phosphonate, so achieving compatibility with the core material. Thus, no formation damage was induced and retention of scale inhibitor by adsorption ensured the potential for long squeeze treatment lifetimes. The modified product has since been applied in the field and results from the laboratory qualification, chemical re-formulation and field treatments are presented in this paper. To date, 27 wells have been squeezed; the part-neutralized chemical targeted at susceptible wells and the acid chemical used elsewhere. Excellent are results in both formation damage and return lifetimes minimising the requirement for coil tubing scale removal treatments in this field.